The recording surface of a magnetic medium such as a tape or disk is generally comprised of ferrous oxide particles of varying sizes and shapes suspended in a polymer base. These particles experience an orienting force via the application of an external magnetic field which is generated by the gap of a magnetic recording head. The external magnetic field will tend to align the magnetic dipoles in the medium relative to the field direction and will result in a remanent magnetization when the field is removed. The coercivity, or field strength required to "coerce" each dipole into realignment, is a function of numerous factors including dipole size, shape, and material. Furthermore, the magnetic particles interact with one another to varying degrees, thereby affecting each other's tendency to reorient when exposed to an external magnetic field. The practical outgrowth of this behavior is that realignment of the magnetic dipoles in the recording medium by the recording head is neither uniform nor instantaneous, and depends on a number of factors including the external field strength and the field strength resulting from the medium itself.
When a binary data stream is written onto a magnetic medium, it is encoded into a series of regions with varying magnetic orientations. For the sake of this discussion, these regions may be described as "positive" and "negative" states which correspond to two differing orientations of the north and south poles of the dipoles contained within the magnetized regions. As a bit stream is serially input to electronics controlling an inductive write head held near a moving magnetic medium, the polarity of the write current is switched back and forth to create this series of magnetized regions.
During the write process, the write signal current flowing through the coil of the write head induces a magnetic field centered over the write head gap. In addition, the magnetization of the media itself produces a local magnetic field which adds to the write head field to produce the net magnetic field strength affecting particle orientation. One such field results from the pole strength of the transition being recorded. An additional field contribution is created by preexisting media magnetization, whether from prior DC biased erasure, or from preexisting data which is being overwritten. The field of the recently encoded transitions as well as the field associated with the previously recorded data may either strengthen or weaken the field generated by the write head such that a timing error will be introduced in the recorded data stream for the affected bits. The effect on the write head (recording) field by the fields generated by previously recorded data and recently recorded transitions is determined by a number of factors, including the polarity of the data/transition poles and their physical proximity to the write head. Accordingly, the fields generated by the transition poles and the prior recorded data produce a resulting field which either enhances or mitigates the write field such that the transition being encoded shifts backward (i.e., in the direction of medium travel) or forward (i.e., opposite the direction of medium travel) in position, respectively. When a forward shifted transition is encountered by the read head, it will produce a resulting signal peak which is shifted behind or lags in time in relation to the remainder of the data since the transition zone does not reach the read head until slightly later. Similarly, a backwards-shifted transition produces a peak with a timing lead when read since it reaches the head earlier.
Timing errors may be reduced by erasing any preexisting data or residual magnetization on the medium. This in theory will eliminate the overall write field contribution from this source. The medium may be erased by applying a decaying high frequency alternating current signal (with peak coercivity substantially in excess of that of the particles in the medium) via an erase head to effectively randomize the magnetic orientations of the medium in order to eliminate the effects of the field generated by preexisting data. This method, however, may have the undesirable side effect of introducing unwanted noise into the recorded signal upon read/playback resulting from a variety of sources including transition noise, under- or over-bias, ac waveform distortion, or anhysteretical recording of recording head flux images. Noise induced by AC erase is especially acute in the newest high coercivity/high particle uniformity tape media such as those of the metal evaporated (ME) and metal particulate (MP) varieties.
A direct current signal may also be utilized to erase the existing data prior to recording the new signal. Unlike the AC method described above, a DC (presaturation) signal is applied via one of the heads in order to create a constant magnetic polarization on the surface of the media. This process has also been found to induce distortion in the recorded signal as a result of DC remnance in the magnetic medium and the creation of a second field as previously described. By erasing with a DC signal biased in one orientation or the other prior to recording, a systematic series of lead and lag errors will be produced on the medium. This results from the contribution of the magnetic field generated by the erased portions of the medium as it approaches the write head (similar to the effect of the previously recorded data discussed above). If the bias is reversed, the systematic errors will merely be inverted, i.e., leads will become lags and vice versa. A system disclosed in U.S. Pat. No. 5,057,948 compensates for the distortional effects resulting from a dc erase field by superimposing a dc magnetic field bias of opposite polarity on the signal to be subsequently recorded. This bias generates a stronger or weaker field in the write gap to offset the remanent erase field. However, that system does not account for variations in the preexisting bias field, and recites no mechanism for determining the appropriate level of bias signal to be superimposed on the data signal being recorded.
It is further noted that as the data encoding rate for a given transport speed of the recording medium (data density) is increased, the width of each transition zone becomes more significant, as does any lead or lag error in the peak associated with each transition. This results primarily from the reduced width of the magnetic region associated with each data bit; a given transition zone width or lead/lag shift will accordingly constitute a larger percentage of the encoded region at data density is increased. When very high data rates or densities are used, some overlap of the read head waveform "events" resulting from adjacent bits occurs. Accordingly, media with relatively narrow distributions of particle coercivity (SFDS) are preferable since they generally allow greater data storage capability while maintaining acceptable system bit error rates (BERs). Similarly, minimization or elimination of peak shift timing errors will allow greater data density while preserving an acceptable BER.
In addition to the problem of signal distortion due to longitudinal shifts in the position or timing of data bit transitions, lateral alignment of the read/write head(s) has also been problematic. If the head is not aligned properly on the track, missing data, signal distortion, and/or excessive crosstalk (i.e., simultaneous reading of more than one track) may result.
Various schemes of lateral head alignment or improvements thereon have been disclosed in the prior art. It is a common feature of all of them, however, that separate head tracking signals must be included in the magnetic medium. For example, U.S. Pat. No. 5,060,092 discloses a system by which a recording medium track is divided into halves, with a first and second tracking signal being applied to the first and second halves, respectively, of every nth track (n being greater than 1) through a bifurcated recording head. Also, U.S. Pat. No. 4,766,508 discloses a floppy disk tracking apparatus which records burst tracking signals in disk sector interstices on alternating sides of a given track (as opposed to the traditional simultaneous recording of burst tracking signals on both fringes of the track through use of two read/record circuit channels). The presence of such tracking signals reduces the amount of recording media available for data storage. In tape cartridges such as the QIC (Quarter Inch Cartridge) format, it is conventional to provide sets of servo tracks, one of which is constantly monitored by one read head while a separate read head scans a data track. In disk drive systems, servo segments are typically embedded between data sectors such that the read head periodically passes over the servo segments when reading data. Typically, two serially adjacent servo signals are provided, one laterally offset from the center of the data track to one side, one laterally offset from the center of the data track to the other side. After the read head passes over these servo segments, the signal strength from the two signals are compared. If the read head is centered over the associated data track, the signal strength from the two servo segments should be equal. The degree of servo signal difference, as well as a determination of which of the two signals is larger provides information concerning the magnitude of read head offset from the center of the track. This information is fed to a motor controller which adjusts the head position back to the center of the data track. In both cases, valuable media area is taken up with tracking information.
Based on the foregoing, it would be highly desirable to provide an improved erasure/recording method by which timing errors in data bit state transitions encoded in the media could be eliminated without the introduction of unwanted noise or distortion inherent in traditional alternating or direct current erase systems, and without imposing further limitations on the data storage capacity of the medium. Furthermore, it would be desirable to provide an improved lateral positioning system of reduced complexity for the read/record head(s) which would provide accurate lateral positioning of the read/write head assembly without relying on dedicated servo information embedded on the media.